Method for reducing cure shrinkage of a thermoset resin

ABSTRACT

A method for reducing cure shrinkage of a thermoset resin includes forming a plurality of surface modified nanofibers. The surface modified nanofibers are formed by soaking nanofibers in an oxidizing acidic solvent. An oxidizing agent is added to the soaking nanofibers, thereby generating heat sufficient for at least one of in-situ oxidation and in-situ exfoliation of a subsurface of each of the nanofibers. Excess oxidizing agent and acidic solvent are removed from the nanofibers, which are then dried. The dried nanofibers have reduced surface hydrophobicity. The surface modified nanofibers are substantially uniformly dispersed into the thermoset resin. The surface modified nanofibers are adapted to reduce cure shrinkage of the thermoset resin during subsequent curing processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/232,710, filed Sep. 22, 2005, now U.S. Pat. No. 7,589,143entitled “Method for Reducing Cure Shrinkage of a Thermoset Resin,”which application is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to polymer composites, and moreparticularly to a method for reducing cure shrinkage of a thermosetresin.

BACKGROUND

Carbon nanofibers have a tendency to interact and entangle themselveswith other carbon nanofibers. This may be due, at least in part, to thephysiochemical properties of the carbon nanofiber surfaces. Further,these properties may also, in some instances, render the nanofibersincompatible with the chemical nature of certain polymeric systems.Problems may arise when attempting to uniformly distribute carbonnanofibers into a polymeric system, due in part, to the potentialagglomeration of the nanofibers and to the potential incompatibility ofthe nanofibers with the polymeric system. Further, the agglomeration ofnanofibers may substantially reduce the effect of the nanofibers asreinforcement fillers in a polymeric system. Still further, theagglomeration of nanofibers may, in some instances, cause local defects,which may lead to reduced mechanical properties of the polymeric system.

As such, it would be desirable to provide nanofibers having reducedfiber-to-fiber interaction and increased compatibility with polymericsystems.

SUMMARY

A method for reducing cure shrinkage of a thermoset resin includesforming a plurality of surface modified nanofibers. The surface modifiednanofibers are formed by soaking nanofibers in an oxidizing acidicsolvent. An oxidizing agent is added to the soaking nanofibers, therebygenerating heat sufficient for at least one of in-situ oxidation andin-situ exfoliation of a subsurface of each of the nanofibers. Excessoxidizing agent and acidic solvent are removed from the nanofibers,which are then dried. The dried nanofibers have reduced surfacehydrophobicity. The surface modified nanofibers are substantiallyuniformly dispersed into the thermoset resin. The surface modifiednanofibers are adapted to reduce cure shrinkage of the thermoset resinduring subsequent curing processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the present disclosure maybecome apparent by reference to the following detailed description anddrawing.

FIG. 1 is a flow diagram depicting an embodiment of a method forreducing cure shrinkage of a thermoset resin;

FIG. 2 is a schematic, cross-sectional view of a reinforced polymericstructure; and

FIG. 3 is a flow diagram depicting a detailed embodiment of a method forreducing cure shrinkage of a thermoset resin.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiment(s) of the surface modified nanofibers disclosed herein have atendency to remain disentangled from each other. Without being bound toany theory, it is believed that the reduced attraction between theindividual fibers advantageously allows the nanofibers to besubstantially uniformly dispersed in a polymer system, thereby producingfiber reinforced polymer composites with improved chemical and/orphysical properties. Non-limitative examples of chemical and/or physicalproperties include increased thermal conductivity, decreased thermalexpansion coefficient, increased electrical conductivity, and/or thelike, and/or increased mechanical strength, modulus, and/or the like.Further, the uniformly dispersed nanofibers may be added to thermosetpolymers to substantially reduce polymer shrinkage during subsequentcuring processes. Non-limitative examples of subsequent curing processesinclude those processes used to achieve desirable dimension stability,surface smoothness, and/or appearance. It is to be understood thatreducing cure shrinkage of a thermoset resin may be desirable in theproduction of composite workpieces having high dimensional precisionand/or high surface quality.

Referring now to FIG. 1, an embodiment of a method for reducing cureshrinkage of a thermoset resin is depicted. Generally, an embodiment ofthe method includes forming a plurality of surface modified nanofibers,as shown at reference numeral 13, and substantially uniformly dispersingthe plurality of surface modified nanofibers into the thermoset resin,as shown at reference numeral 15. It is to be understood that theplurality of surface modified nanofibers advantageously assists inreducing cure shrinkage of the thermoset resin during subsequent curingprocesses.

In an embodiment, the plurality of surface modified nanofibers is formedby soaking the nanofiber(s) in an oxidizing acidic solvent. Any suitablenanofiber(s) may be selected. In an embodiment, the nanofibers arecarbon nanofibers. A non-limitative example of suitable nanofibersincludes vapor grown carbon nanofibers.

The nanofibers may be soaked in the oxidizing acidic solvent for apredetermined amount of time. In an embodiment, the predetermined amountof time ranges from about 1 minute to about 60 minutes. In an alternateembodiment, the predetermined amount of time is less than about 5minutes. It is to be understood that any suitable acidic solvent may beused to soak the nanofiber(s). Non-limitative examples of such oxidizingacidic solvents include sulfuric acid (a non-limitative example of whichis sulfuric acid with a molarity exceeding 14), perchlorates, oxo-acids,peroxides, and/or combinations thereof.

An oxidizing agent is added to the soaking nanofibers. It is to beunderstood that the oxidizing agent advantageously generates heatsufficient for initiating in-situ oxidation and/or in-situ exfoliationof a subsurface of each of the nanofibers. It is to be furtherunderstood that the oxidizing agent may also generate polarfunctionality of the nanofiber surfaces.

Non-limitative examples of oxidizing agents include at least one ofperoxides, superoxides of transition metals (non-limitative examples ofwhich include those transition metals having an oxidation state greaterthan about 3 valence states), superoxides of non-transition metals(non-limitative examples of which include those non-transition metalshaving an oxidation state greater than about 3 valence states),polyanions, polyoxometalates, and combinations thereof. Examples ofsuitable superoxides of transition metals include, but are not limitedto iron oxides, chromium oxides, manganese oxides, and/or combinationsthereof. Examples of suitable polyanions include, but are not limitedto, permanganates, tungstates, and/or molybdates, and/or combinationsthereof.

In an embodiment of the method, the nanofiber(s) may be soaked in waterprior to being soaked in the oxidizing acidic solvent, and/or prior tothe addition of the oxidizing agent. In a non-limitative example, thenanofiber(s) are soaked in water, and the oxidizing agent is added priorto the addition of the oxidizing acidic solvent.

After and/or during the oxidation and/or exfoliation of the nanofibers,surface modifying agents (non-limitative examples of which includepolyalcohols, polyesters, polyethers, and/or the like, and/orcombinations thereof) may optionally be added to the soaking nanofibers.

Excess oxidizing agent and acidic solvent may then be removed. In anembodiment, removal of the agent and solvent is accomplished by rinsingthe nanofibers with an aqueous solvent (non-limitative examples of whichinclude water, a water and polyalcohol mixture, a water and polyethyleneoxide mixtures, and combinations thereof).

In an embodiment of the method, a surfactant (e.g. a surfactantsolution) may also be added to the nanofibers. It is to be understoodthat suitable surfactants include, but are not limited to, those havingmolecules and/or polymers that are compatible with the treatednanofibers and/or the polymer matrix (described further hereinbelow).Non-limitative examples of such surfactants include polyethylene oxide,polyethers, polyesters, glycols, and/or combinations thereof.

The rinsed nanofibers may then be dried. Drying may be accomplished at atemperature ranging from about 50° C. to about 110° C. for a timeranging from about 4 hours to about 24 hours.

The surface modified nanofibers are then added to a thermoset resin. Thethermoset resin may include a polymer matrix, a non-limitative exampleof which includes an epoxy resin. Other examples of suitable thermosetresins include, but are not limited to polyurethanes, phenolic resins,phenolic-formaldhyde polymers, two component epoxy resins, acrylics,polyimides, polyalkylsilicones, and/or combinations thereof.

The modified surface of the nanofibers assists in the substantiallyuniform distribution of the fibers throughout the thermoset resin. It isto be understood that the resulting surface treated/modified nanofibershave a reduced surface hydrophobicity, when compared to non-surfacemodified nanofibers. Without being bound to any theory, it is believedthat the modified surface nanofibers (having reduced hydrophobicity)advantageously substantially lack the tendency to entangle themselvestogether, thereby allowing for substantially uniform distribution.

In an embodiment, the surface modified nanofibers are mixed in aninterface polymer prior to being added to the thermoset resin. It is tobe understood that the interface polymer is generally compatible withboth the surface modified nanofibers and the thermoset resin, therebyadvantageously contributing to the uniform distribution of the surfacemodified nanofibers throughout the thermoset resin. Non-limitativeexamples of an interface polymer include polymers containing heteroatoms(such as, for example, oxygen and nitrogen), polyethylene oxides,polypropylene oxides, polyimides, copolymers thereof, and/orcombinations thereof.

Referring now to FIG. 2, an embodiment of a reinforced polymericstructure 10 is depicted. In this non-limitative embodiment, one surfacemodified nanofiber 12 is shown dispersed in the thermoset resin 14 forillustrative purposes. It is to be understood that many nanofibers 12may be substantially uniformly dispersed throughout the thermoset resin14. In this embodiment, the nanofiber 12 is shown having the interfacepolymer 16 established thereon.

FIG. 3 illustrates an embodiment of a method of forming a workpieceincorporating the surface-modified nanofibers 12 therein. Carbonnanofibers (CNF), as shown at reference numeral 17, are added to andmixed in water, as shown at reference numeral 19. An oxidizing agent andadditional water are added to the water/nanofiber mixture, as depictedat reference numeral 21. Acid is mixed into thewater/nanofiber/oxidizing agent mixture, as shown at reference numeral23. This embodiment of the method includes adding a surface modifyingagent to the acid/water/nanofiber/oxidizing agent mixture, as shown atreference numeral 25.

The mixture is then washed and filtered, as depicted at referencenumeral 27, and then dried, as depicted at reference numeral 29. It isto be understood that the washing, filtering, and drying steps may berepeated as desired, as indicated by the forward and reverse arrowsbetween boxes 27, 29.

In this non-limitative embodiment, an epoxy is added to the driedmixture, as shown at reference numeral 31.

Depending, at least in part, on the workpiece to be formed, the methodmay include adding additional epoxy, hardeners, and/or catalysts (asshown at reference numeral 33) to the epoxy mixture formed at referencenumeral 31. The resulting mixture may be cured in a mold, as depicted atreference numeral 35; thereby forming the workpiece, as depicted atreference numeral 37.

To further illustrate embodiment(s) of the present disclosure, thefollowing examples are given. It is to be understood that these examplesare provided for illustrative purposes and are not to be construed aslimiting the scope of embodiment(s) of the present disclosure.

EXAMPLE 1

About 50 grams of carbon nanofibers (commercially available under thetradename PYROGRAF III from Applied Sciences, Inc., located inCedarville, Ohio) is mixed with water (ranging from about 60 ml to about100 ml). The mixture is stirred for about 5 to 10 minutes. About 10grams of chrome trioxide is mixed into the carbon nanofiber/watermixture by adding CrO₃ powder or an aqueous solution of chrome trioxide(e.g. 25 ml of 4 mole CrO₃ solution). This solution is stirredvigorously for about 5 to about 10 minutes to increase the wetting onthe carbon nanofibers.

The carbon nanofibers are mixed with and soaked in about 150 ml to 200ml of sulfuric acid for about 5 minutes. About 100 ml to 200 ml of waterare mixed in the acid soaked nanofibers for another 5 minutes. Themixture is filtered through a paper sieve, and the filtrate is collectedafter filtration or after a mild washing with water.

A surfactant solution is added to the filtrate and is mixedsubstantially continuously for about 10 to about 30 minutes. Thismixture is filtered. The treated carbon nanfibers are dried at atemperature ranging from about 50° C. to about 110° C. for a timeranging from about 4 hours to about 24 hours.

The dried carbon nanofibers are added into a warm epoxy resin to make amaster batch having about 5 wt. % to about 10 wt. % surface modifiednanofibers. The master batch is stirred at 2000 rpm to 5000 rpm, using alaboratory intensive mixer, for about 5 minutes to about 15 minutes attemperatures ranging from room temperature to about 50° C.

EXAMPLE 2

An epoxy formulation included 100 parts by weight of diglycidyl etherbisphenol-A resin (commercially available under the tradename DER 383from The Dow Chemical Company, located in Midland, Mich.); 80 parts byweight of methyltetrahydrophthalic anhydride hardener (commerciallyavailable from Lonza, located in Allendale, N.J.); and 2 parts by weightof 1,2-dimethyl imidazole catalyst (commercially available under thetradename DMI from BASF Corp., located in Florham Park, N.J.). Allsamples (each having a different amount of surface modified carbonnanofibers) were cast in a 12″×12″×0.125″ mold and cured in a hot airoven for about 20 minutes at 90° C., followed by post curing for about 2hours at 150° C. The following table exhibits cure shrinkage volumepercent of the various samples including different amounts of surfacemodified carbon nanofibers.

TABLE 1 Carbon Cure Nanofiber Shrinkage SAMPLE (Wt. %) Carbon NanofiberDescription (Vol. %) 1 0.0 — 1.61 2 3.0 As received, no modification1.38 3 ~5 dried, acidic, 10% masterbatch 0.75 4 ~5 wet, acidic, 10%masterbatch 1.28 5 ~3 wet, acidic, 10% masterbatch 1.19 6 3.0 dried,acidic, 10% masterbatch 0.19 7 2.1 dried, acidic, 4% masterbatch 0.52

As depicted, the volume shrinkage of the samples (3-7) having surfacemodified carbon nanofibers is less than the sample (1) having no surfacemodified carbon nanofibers and the sample (2) having 3% unmodifiedcarbon nanofibers. Further, the dried surface modified carbon nanofibersappeared to be more effective in reducing the cure shrinkage of thesamples than the wet surface modified carbon nanofibers.

Embodiment(s) of the surface modified nanofibers include, but are notlimited to the following advantages. It is believed that reducedinteraction between the individual nanofibers advantageously allows thenanofibers to be substantially uniformly dispersed in a polymer system,thereby producing fiber reinforced polymer composites with improvedchemical and/or physical properties. Further, the uniformly dispersednanofibers may be added to thermoset polymers to substantially reducepolymer shrinkage during subsequent curing processes.

While several embodiments have been described in detail, it will beapparent to those skilled in the art that the disclosed embodiments maybe modified. Therefore, the foregoing description is to be consideredexemplary rather than limiting.

1. A method for making a surface modified nanofiber, the methodcomprising: soaking a nanofiber in an oxidizing acidic solvent; addingan oxidizing agent to the soaking nanofiber, thereby generating heatsufficient for at least one of in-situ oxidation and in-situ exfoliationof a subsurface of the nanofiber; removing excess oxidizing agent andoxidizing acidic solvent from the nanofiber; and drying the nanofiber,thereby forming the surface modified nanofiber having reduced surfacehydrophobicity, the surface modified nanofiber adapted to reduce cureshrinkage of a thermoset resin into which it is incorporated.
 2. Themethod as defined in claim 1 wherein the oxidizing agent is a transitionmetal in an oxidation state greater than about 3 valence states.
 3. Themethod as defined in claim 1 wherein prior to soaking the plurality ofnanofibers in the oxidizing acidic solvent and adding the oxidizingagent, the plurality of nanofibers is soaked in water.
 4. The method asdefined in claim 1 wherein soaking the nanofiber in the oxidizing acidicsolvent is accomplished for a time ranging from about 1 minute to about60 minutes.
 5. The method as defined in claim 1, further comprisingmixing the nanofiber with a surfactant solution prior to drying thenanofiber.
 6. The method as defined in claim 1 wherein the nanofiber isa carbon nanofiber.
 7. The method as defined in claim 1 wherein removingexcess oxidizing agent and oxidizing acidic solvent is accomplished byrinsing the soaking nanofiber having the oxidizing agent therein withwater.
 8. The method as defined in claim 1, further comprising: mixingthe surface modified nanofiber in an interface polymer; andsubstantially uniformly mixing the surface modified nanofiber in thethermoset resin.